1. Field of the Invention
This invention relates to an upper-feed type sewing machine with an upper feed dog to clamp and feed cloth as a workpiece in cooperation with a lower feed dog.
2. Description of Related Art
In a sewing machine, a workpiece to be sewn is held between a presser foot and a needle plate. The pressor foot is mounted to a machine head and extends across a needle path (or a needle drop position). The needle path plate is placed on a machine bed. Generally, feeding the workpiece is achieved by a lower feed dog provided below the needle plate so as to impart a feed motion to the workpiece. During operation of the machine, a known lower feed mechanism is effective to impart to the lower feed dog a predetermined motion, typically, an elliptical motion in a vertical plane along workpiece direction of travel. The lower feed dog, when at the upper portion of the elliptical motion thereof, rises above the needle plate and then, cooperates with the presser foot to clamp and feed the workpiece.
A disadvantage of the arrangement thus constructed is that the lower feed dog is effective to impart a feeding motion to the only underside of the workpiece. For example, where a plurality of plies of material are superimposed, it is likely that the lower workpiece to which a feeding motion is imparted is displaced from the upper workpiece which slides on the presser foot. To obtain a high quality product, an operator is required to finely adjust the advancement of the material.
To that end, an upper feed mechanism is incorporated into sewing machines such as a covering chain stitch sewing machine and a multi-thread chain stitch sewing machine wherein a plurality of superimposed workpieces are stitched together. The upper feed mechanism includes a feed dog (upper feed dog) provided above the needle plate and located forwardly of the path of a needle. The upper and lower feed dogs are operatively associated with one another to clamp the workpieces on the needle plate. A feeding motion is conducted on opposite sides of the workpieces.
FIG. 4 is a side view, in section, of a part of a conventional sewing machine with such an upper feed mechanism, located adjacent to the path of a needle. In FIG. 4, A designates a sewing machine head, and B designates a sewing machine bed. A needle plate C is placed on the bed B and is situated below the head A. The sewing machine is operable to form stitches while a workpiece is fed across the needle plate C and moved to the left in FIG. 4.
A needle bar 54, a presser bar 55, and an upper feed bar 56 each extend vertically downwardly toward the needle plate C and are juxtaposed in this order in the direction in which workpieces W1 and W2 are fed.
The needle bar 54 is supported by a bushing 57 which is, in turn, fixedly mounted within the machine head A and vertically slidable within the bushing 57. A needle 58 is secured to the lower end of the needle bar 54 and moved up and down by means of a main shaft (not shown) within the machine head A. Three (or two) needles 58 may be provided such that they are arranged perpendicular to the feeding direction, and the corresponding number of the needle bars 54 may be provided.
The presser bar 55 located downstream from the needle bar 54 is also supported by a bushing 59 which is, in turn, fixedly mounted to the machine head A and vertically slidable within the bushing 59. A presser foot 62 is mounted to the lower end of the presser bar 55 via a presser holder 60 and a presser arm 61. The presser holder 60 is secured to the lower end of the presser bar 55. The presser arm 61 has an upper end mounted to the presser holder 60 at a position offset from the central axis of the presser holder 60 and swingable about a pivot pin 63. The presser foot 62 is swingably mounted to the lower end of the presser arm 61 through a pivot pin 64. The presser bar 55 has a hollow structure to receive a presser spring 65. The presser spring 65 has an upper end engaged with the lower end of a threaded stopper (not shown) threaded into the upper end of the presser bar 55. Also, the lower end of the presser spring 65 is engaged with the upper end of the presser arm 60.
With this arrangement, the presser foot 62 is urged downwardly against the needle plate C under the action of the presser spring 65. When the workpieces W1 and W2 are fed on the needle plate C, and when an upward force is applied through the workpieces to the presser foot 62 against the action of the presser spring 65, the presser arm 61 is swung about the pivot pin 63 to cause the presser foot 62 to cooperate with the upper surface of the needle plate C to clamp the workpieces W1 and W2. At this time, the presser foot 62 is swung about the pivot pin 64 so as to change its angle of inclination relative to the presser arm 61. Thus, the workpieces W1 and W2 can constantly be clamped between the needle plate C and the entire upper surface of the presser foot 62.
The upper feed bar 56 is located downstream the presser bar 55 and has a central portion extending through a bushing 66. The upper feed bar 56 is manually axially movable within the bushing 66. The bushing 66 is carried by the machine head A and swingable back and forth. A feed element 68 is mounted to the lower end of the upper feed bar 56 through a feed holder 67. The feed element 68 is an elongated plate-like member and has a rear end pivotally mounted to the feed holder 67 through a pivot pin 69. The feed element 68 extends on one side of each of the presser foot 62 and the needle 68 and terminates at an upper feed dog 70. The upper feed dog 70 is located forwardly of the needle 58 and confronts the upper surface of the needle plate C.
The upper end of the upper feed bar 56 is connected through a known transmission mechanism to a main shaft (not shown) within the machine head A. Rotation of the main shaft causes simultaneous swing and axial motions of the upper feed bar 56 within the bushing 66. The combined swing and axial motions then give the lower end of the upper feed bar 56 an elliptical motion in a plane in which the central axis of the upper feed bar 56 is included and in the direction in which the workpieces W1 and W2 are fed. As a result an elliptical motion is imparted to the feed dog 70 mounted to the distal end of the feed element 68 which is, in turn, mounted to the lower end of the upper feed bar 56.
The upper feed dog 70 and a lower feed dog 71 below the needle plate C are driven both through an elliptical path, but in opposite directions. When the upper feed dog 70 is moved down, the lower feed dog 71 is moved up or closer to the upper feed dog 70. Thereafter, the upper feed dog 70 and the lower feed dog 71 are both moved to the rear in the direction of workpiece travel. This arrangement allows the upper feed dog 70 and the lower feed dog 71 to clamp the workpieces W1 and W2 fed on the needle plate C. Thus, a feed motion is imparted to opposite sides of the workpieces to properly move the workpieces to a needle path and eliminate displacement between the lower workpiece W2 and the upper workpiece W1 as stated above.
This type of sewing machine includes an additional lower feed dog 72 behind the lower feed dog 71 which cooperate with the upper feed dog 70 to impart a feed motion to the workpieces W1 and W2. The lower feed dog 72 is moved with the lower feed dog 71 to impart a feed motion to the workpieces W1 and W2 held between the presser foot 62 and the lower feed dog. This arrangement ensures proper advancement of the workpieces.
However, in the conventional sewing machine thus constructed, as the upper feed bar 56 is located behind the presser bar 55 as seen from the needle bar 54, the fulcrum (pivot pin 69) of the feed element 68 carried by the lower end of the upper feed bar 56 is located furthest away from the upper feed dog 70 or a working point. This results in an increase in the length and size of the feed element 68. Accordingly, its moment increases. In such a case, the upper feed dog 70 cannot be moved in response to the motion of the lower feed dog 71 when the sewing machine is operated at high speeds. Also, the upper feed dog 70 is subjected to bounce, and thus creating difficulty in obtaining a proper feed force.
To prevent the bouncing of the upper feed dog 70, the force of a spring (not shown) disposed on the presser bar 56 could be increased. In such a case, however, the upper feed dog 70 and the lower feed dog 71 may unduly clamp and cause damage to workpieces being sewn. Care should therefore be taken when the conventional sewing machine is used to sew knitted or other fragile garments at high speeds.
To form a flat seam, the sewing machine of this type is required to include a spreader mechanism with a swing arm adapted to feed a cover thread to a location immediately before the path of a needle. The cover thread is interwoven between and connects a plurality of needle threads. However, it is difficult to arrange a drive shaft for use in the swing arm of the spreader mechanism in such a location as to form an appropriate seam because the moving components of the presser and upper mechanisms are closely arranged in that location.